Alignment-tolerant lens structures for acoustic force actuation of cantilevers

ABSTRACT

In accordance with one aspect of the present invention, a cantilever of a probe-based instrument is deflected by directing a beam of ultrasonic acoustic energy at the cantilever to apply acoustic radiation pressure to the cantilever. The energy is generated by an acoustic actuator. The transmitted beam preferably is focused using a cylindrical lens, providing a beam tightly focused in one dimension and unfocused in a second dimension. In accordance with another aspect of the present invention, a power source such as an RF signal generator is operated so as to spread the spectrum of acoustic radiation on a time scale that is short or comparable to the acoustic roundtrip time. Such a design diminishing the resonance effects and sensitivity to spacing between the cantilever and the acoustic source.

This application claims the benefit of U.S. patent application Ser. No.10/323,384, filed Dec. 18, 2002, the disclosure of which is incorporatedherein in its entirety, by reference.

BACKGROUND OF THE INVENTION

The present application is directed to driving systems, and moreparticularly to a method and apparatus for driving a cantilever throughthe use of acoustic wave pressure generated by an ultrasonic actuator.

Scanning probe microscopes (SPMs) represent a category of probe-basedinstruments designed to characterize a surface of a sample at an atomiclevel, through the monitoring of an interaction between a sample and atip on a cantilever probe. This interaction is primarily a scanningoperation between the tip and sample, whereby data regardingcharacteristics of the surface is acquired and used to generate an imageof the sample region. The image data is commonly acquired via a rasterscan of the sample.

A particular type of SPM is known as an atomic force microscope (AFM),which functions by measuring local properties of a sample, such asheight, optical absorption, magnetism or other measurablecharacteristic. The resulting image will resemble an image on atelevision screen, as it consists of both many rows or lines ofinformation placed one above the other.

AFMs are designed to operate in a variety of modes, includingnon-contact mode, contact mode and oscillating mode, also known as atapping mode. In the non-contact mode, the AFM generates a topographicimage from measurements based on attractive forces. In this design, thetip does not touch the sample. The non-contact mode does not functioneffectively in liquids.

In the contact mode of operation, the AFM scans the tip across thesurface of the sample, while the force of the tip on the surface of thesample is maintained at a generally constant value. This contactoperation is accomplished by moving either the sample or the probeassembly vertically to the surface of the sample upon sensing adeflection of the cantilever as the probe is scanned horizontally acrossthe surface.

In the oscillating mode, the tip is made to oscillate at or near aresonant frequency of the cantilever. The amplitude or phase of thisoscillation is kept constant during scanning using feedback signals,which are generated in response to tip-sample interaction. Similar tothe contact mode, the feedback signals are then collected, stored andused as data to generate an image of the sample area.

Atomic force microscopes are known to have a resolution down to theatomic level for a wide variety of surfaces. While the general conceptof an AFM is similar to that of a record player, as well as the stylusprofilometer, to obtain and enable the atomic-scale resolution, AFMshave refinements, including an optical sensor which operates byreflecting a laser beam off of the cantilever. Angular deflection of thecantilever causes an angular deflection of the laser beam. The reflectedlaser beam strikes a position detector to indicate the position of thelaser spot on the detector, and thus the angular deflection of thecantilever.

An area of particular interest in designing an AFM is the mechanismemployed to provide an external force to deflect or oscillate thecantilever. In existing AFMs, the cantilever will typically beoscillated by a piezoelectric actuation.

Traditional piezoelectric drives act on the base of the cantilever, noton a free-end portion. Therefore, these systems must apply substantiallygreater forces to the cantilever to obtain a given deflection magnitudeat the free end than would be required if force were applied directly tothe free end or the body of the cantilever. Such a design results incertain limitations.

Since a typical AFM cantilever is easily excited to resonance in air,the piezoelectric drive is useful in this environment. However,piezoelectric drives are not very useful in liquid (e.g., water)environments. The reason for this has to do with the quality factor or Qof a resonance of the cantilever. The quality factor, Q, denotes thesharpness of a cantilever's resonance curve. A resonance with a large Qcan be excited to relatively large cantilever oscillation amplitudeswith relatively small excitation forces. For operation in air or othergaseous environments, the typical piezoelectric drive provides ampleexcitation force to drive the cantilever to produce a resonance peakthat is easily identified and distinguished from parasitic resonancepeaks, such as those of the mounts for the cantilever and thepiezoelectric drive itself.

Conversely, a cantilever operated in liquids such as water, has adramatically lower Q, as the liquid dampens the oscillating cantilever.A typical piezoelectric drive does not have enough gain to excite thecantilever sufficiently to produce a resonance peak that is easilylocated and differentiated from parasitic resonances.

In view of this, specialized cantilever drives have been developed toact along the length of the cantilever rather than only on the base. Onesuch drive is known as a magnetic drive. The typical magnetic drive hasa magnetic cantilever that is driven by an electromagnetic force. Thecantilever has a fixed base rigidly attached to a support and bears atip on its free end that interacts with a sample. The cantilever isrendered magnetic by coating one or more of its surfaces with a magneticlayer. By controlling the amplitude of the applied magnetic field, thecantilever can be deflected while the tip interacts with the sample.

However, a magnetic drive has inherent limitations that considerablyrestrict its range of applications. For instance, it requires a specialmagnetically-coated cantilever and cannot therefore be used inapplications where the cantilever should not be coated with magneticmaterial. It is also not applicable to situations where the magneticproperties of the sample and/or the environment results in undesirabledeflection of the cantilever, producing errors in the measurements. Theoperating ranges of the magnetic drive system are also limited.

An acoustic drive has also been considered to drive the cantilever. Inthis design, a cantilever and piezoelectric drive are mounted on acommon head in a spaced-apart relationship. The head is mounted above afluid cell, and the cantilever extends into the fluid cell to interactwith the sample in the cell. The piezoelectric drive can be excited by asignal generator to generate acoustic waves that propagate through theglass walls of the fluid cell, through the fluid in the cell, then on tothe cantilever, causing the cantilever to oscillate.

Acoustic drives of this type have various disadvantages. For instance,the unfocused acoustic energy will impinge on many other components ofthe system, such as mounts for the cantilever and the piezoelectricdrive, the fluid cell, and even the fluid, exciting resonances in thosecomponents. These resonances can be difficult to distinguish from thecantilever resonance. The acoustic drive also has sufficient actuationforce at a limited selection of operation frequencies and can be achallenge to match the cantilever resonance with the operation frequencyof the acoustic actuator. If a user selects a resonance that does notoverlap with the cantilever resonance, the measurements may be unstable.

Pending application U.S. Ser. No. 10/456,136 (Publication No. US2004-0020279 A1, entitled “Method and Apparatus for the UltrasonicActuation of the Cantilever of a Probe-Based Instrument”; U.S. Pat. No.6,779,387 (U.S. Ser. No. 10/095,850 (Publication No. US 2003-0041657A1)), entitled “Method and Apparatus for the Ultrasonic Actuation of theCantilever of a Probe-Based Instrument”; and U.S. Pat. No. 6,694,817(U.S. Ser. No. 10/096,367 (Publication No. US2003-0041669A1)), entitled“Method and Apparatus for the Ultrasonic Actuation of the Cantilever ofa Probe-Based Instrument”, (all claiming priority to provisional patentapplication Ser. No. 60/313,911, filed Aug. 21, 2001) (commonlyassigned) (all hereby incorporated by reference), describes anultrasonic force microscope (UFM) intended to have an actuator thatdrives a cantilever to produce a “clean” frequency response, preferablyby driving the cantilever body, rather than the base. It is stated thatby driving the body of the cantilever with an ultrasonic actuator, amuch higher localized force can be achieved through the use of atraditional piezoelectric actuator. The beam used for actuation ispreferably shaped, i.e., manipulated to limit unwanted propagation anddirections other than toward the cantilever, so that ultrasonic energyimpinges at least primarily on the cantilever.

Two suitable techniques for shaping the beam are listed as focusing andcollimation. The ultrasonic small diameter beams can be focused on thecantilever using a Fresnel lens or other focusing device located betweenthe ultrasonic actuator and the cantilever. It is noted that the Fresnellens may focus the ultrasonic beam to a diameter of approximately 5 μmto 10 μm at a focal distance of 360 μm, where the 5 μm diameter is evensmaller than the 8 μm to 12 μm diameter of most laser beams. As aresult, it is stated the lens can be used to apply a pinpoint force tothe free end of the cantilever or any other point of interest along thelength of the cantilever.

It is also proposed that in an alternative embodiment, the beam may beintentionally sized larger than the cantilever to account for tolerancesin alignment of the cantilever and the ultrasonic actuator. If, forexample, the cantilever is 50 microns wide, and can be reproduciblyaligned within ±100 microns, an ultrasonic actuator with a beam width of250-300 μm in the region of the cantilever could guarantee that aportion of the ultrasonic beam would always strike the cantilever.

Thus, in the incorporated applications, the preferred embodimentpurports to disclose an apparatus and procedure for providing pinpointactuation energy to a cantilever. It is also acknowledged that alignmenterrors may exist between the cantilever and actuator, whereby thepinpoint accuracy may result in the acoustic beam not impacting thecantilever.

Misalignment of a probe may occur during manufacture or when thecantilever probe is replaced. Particularly, it is known that inoperation, the tip carried on the cantilever becomes damaged or worn andwill require replacement. Normally, the cantilever and tip come as asingle unit, and the entire unit is replaced with a new cantilever/tiparrangement. This replacement operation is a mechanical operation, and adegree of imprecision in the alignment procedure exists. Therefore, whenthe cantilever/tip arrangement is inserted, and a focused pinpoint(i.e., small diameter) beam is used, misalignment may result in improperinteraction between the actuator and the cantilever. To address thisissue it is proposed that the beam (i.e., the diameter) is enlarged.

From the foregoing, it can be seen there are concerns related to use ofa pinpoint acoustic beam due to misalignment issues. A further issue,however, is that widening the beam to address misalignment causes anincrease in undesirable reflections and resonance between the acousticsource and the cantilever. These resonances can result in strongvariations in the acoustic force delivered to the cantilever as theresonance conditions vary with variations in spacing between thecantilever and acoustic source.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one aspect of the present invention, a cantilever ofa probe-based instrument is deflected by directing a beam of ultrasonicacoustic energy at the cantilever to apply acoustic radiation pressureto the cantilever. The energy is generated by an acoustic actuator. Thetransmitted beam preferably is focused using a cylindrical lens,providing a beam tightly focused in one dimension and unfocused in asecond dimension. In accordance with another aspect of the presentinvention, a power source such as an RF signal generator is operated soas to spread the spectrum of acoustic radiation on a time scale that isshort or comparable to the acoustic roundtrip time. Such a designdiminishes the resonance effects sensitivity to spacing between thecantilever and the acoustic source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an acoustic ultrasonically actuated AFMconstructed in accordance with a concept of the present invention;

FIG. 2A is a side view of a cantilever and acoustic wave interaction;

FIG. 2B is a bottom view of FIG. 2A;

FIG. 2C depicts a misalignment between the cantilever and acoustic wave;

FIG. 3A depicts an enlarged acoustic beam impinging energy to acantilever;

FIG. 3B depicts a bottom view of FIG. 3A;

FIG. 4 schematically illustrates a beam produced by a cylindrical orlinear Fresnel lens;

FIG. 5A is a side view of a cantilever and acoustic beam generated by acylindrical lens of FIG. 4;

FIG. 5B is a bottom view of FIG. 5A;

FIG. 6 is a sectional view of an acoustic actuator system according toFIG. 1, driven by a dual frequency RF pulse generator to suppresshalf-wave resonances in accordance with one aspect of the presentapplication;

FIG. 7 is a sectional view of an acoustic actuator driven bymulti-frequency RF pulses to suppress half-wave resonances;

FIG. 8 schematically illustrates an acoustic ultrasonically actuated AFMconstructed in accordance with a further embodiment of the applicationin which the AFM is configured for tapping mode operation;

FIG. 9 is a schematic top view of the acoustic ultrasonic drive for anAFM constructed in accordance with yet a further embodiment of theinvention in which the acoustic ultrasonic actuator and detector arepositioned on a common side of the cantilever;

FIG. 10 schematically illustrates an acoustic ultrasonically actuatedAFM constructed in accordance with a further embodiment of the inventionin which the AFM is configured to take elasticity measurements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an AFM 10 which incorporates a cantileverdeflecting system according to the present application is illustrated.The AFM 10 includes a probe 12 configured to operate in a liquidcontained within a cell 14, an ultrasonic actuator 16 for generatingacoustic waves, and a detector system (18, 20). Probe 12 includes acantilever 22 and a tip 24 mounted or otherwise provided on a free endof the cantilever 22. The cantilever, in one embodiment, is made ofsilicon and may have any of among a number of spring constants andcorresponding fundamental resonances. In one example, a cantileverhaving a spring constant of 0.148 N/m will have a fundamental resonancearound 4.6 kHz in water.

The base of the cantilever 22 is fixed to an optically transparent probesupport 26, which may fit into a commercial AFM scanhead 28. The fluidcell 14 is positioned beneath the scanhead 28 with the probe 12 closelylocated. An ultrasonically transparent substrate 30, preferably made ofa hard substrate like glass or silicon, is placed below the probe 12 andsupports fluid cell 14. The ultrasonic actuator 16, which is preferablyformed from a zinc oxide (ZnO) transducer, is mounted on the bottom ofthe substrate 30, although other materials which will generate theappropriate output may also be used. Alternatively, the ultrasonicactuator can be placed on the side of the substrate directly facing thecantilever. The ultrasonic transducer can then be shaped as a focusingdevice and electrically isolated from the fluid environment, removingthe need for an ultrasonically transparent substrate.

The ultrasonic actuator 16 is driven by a RF signal generator 32 togenerate a beam 34, focused by a lens 36 that deflects the cantilever 22away from the substrate 30. The RF signal generator 32 has an optionalmodulation input that allows the amplitude of the RF signal to be variedwith time. The modulation signal may be a square wave, a sinusoidalwave, a triangle wave or an arbitrary time-varying modulation. The RFsignal generator also has an input or an internal adjustment that allowscontrol over the baseline (unmodulated) power of the RF signal.

The scanhead 28 may include an XY actuator and a Z actuator to permitthe probe 12 to scan a sample (not shown) in the fluid cell 14.Alternatively, the scanhead 28 could be stationary, and the substrate 30could be driven to scan relative to the scanhead 28.

Detector system 18, 20 detects cantilever deflection during scanning.The detector system includes a laser 18 positioned above cantilever 22and a photodetector system 20 configured to receive laser lightreflected from the upper surface of the cantilever 22. As isconventional, signals from the photodetector 20 can be used as feedbackto control operation of the RF signal generator 32 to maintain a desiredcharacteristic of cantilever deflection, such as magnitude, and/or phaseduring scanning.

It is known that a plane target placed in the path of an acoustic wavebeam in an unconfined medium experiences a time averaged force per unitarea. This pressure is known as the “Langevin acoustic radiationpressure” (ARP). Forces imposed by the application of this pressure areused to deflect the cantilever of the probe-based-instrument, such asprobe 12. The ARP is related to the average energy density, U, at thetarget surface. As a simple model based on plane waves, it can beassumed that, at the focal plane, a time harmonic acoustic pressure waveof amplitude, Pi, is normally incident on a cantilever immersed in aliquid and that the wave is reflected with a complex pressure reflectioncoefficient, Γ. This reflection coefficient can be considered as aweighted average over the incident spectrum of plane waves that would beincluded in a focused beam. In this case, the time-averaged energydensity at the cantilever surface will be given by

$\begin{matrix}{U = {\frac{P_{i}^{2}}{2\rho\; c^{2}}\left( {2{\Gamma }^{2}} \right)}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$

where ρ is the bulk density of fluid, c is the speed of sound in theliquid and Γ denotes the absolute value of the reflection coefficient.Using the relation that the average intensity of the incident beam isgiven by I_(I)=P_(i) ²/(2ρc), the Langevin radiation pressure on thecantilever 22, Ω, can be expressed in terms of the intensity as

$\begin{matrix}{\Omega = {\frac{I_{i}}{c}\left( {2{\Gamma }^{2}} \right)}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$The total force applied to the cantilever in the direction ofpropagation of the incident waves can be found by integrating theradiation pressure. Accordingly, the total applied force is proportionalto the average power incident on the cantilever 22. Note that thediscussion above neglects the absorption of the ultrasonic energy in thebeam and in the fluid medium. In case of absorption in the fluid medium,acoustic streaming can be induced. The fluid flow induced by thismechanism can generate additional forces on the cantilever. The lossesin the cantilever are generally very small and hence can be neglected.Also note that Equations 1 and 2 apply to cantilever actuationapplications in air. Since the velocity of sound in air is approximately330 m/s and Γ˜I, the same amount of force can be applied to thecantilever with ⅕^(th) of the acoustic power.

As previously mentioned, and now more particularly illustrated in FIGS.2A and 2B, in existing systems a focused acoustic beam 40 may begenerated with an extremely small diameter by use of a circular Fresnellens to provide a pinpoint beam or spot on a free end of cantilever 42(i.e., such as near a tip 44). When alignment is proper, sufficientenergy is imparted to deflect the cantilever in an appropriate manner.However, as shown in FIG. 2C, when misalignment exists, acoustic beam 40does not impart its energy to the cantilever 42.

An alternative operation to avoid this issue is to widen the beam, suchas for example shown in FIGS. 3A and 3B. However, an issue created bythis alternative is that while acoustic beam 50 imparts energy tocantilever 52, it will also generate undesirable reflected acousticsignals 54 which can cause strongly varying forces on the cantilever forvery small changes in the spacing between the cantilever and acousticsource.

To address these issues, in one embodiment, lens 36 of FIG. 1 is acylindrical lens, such as a Fresnel lens comprised of linear segments,micro-machined into the surface of substrate 30. Lens 36 of thisembodiment, as shown in FIG. 4, generates a linear beam 60 (shown inother view as beam 34) tightly focused in one dimension and unfocused inanother. As a result, lens 36 applies a force to the free end of thecantilever 22 or any other area of interest along the length of thecantilever 22. Of course, the focal length of lens 36 can be varied toaccommodate any physical design constraints to place the actuatorfurther or closer from the cantilever.

By implementing a cylindrical lens with AFM 10, it is ensured, asillustrated in FIGS. 5A and 5B, that the linear acoustic beam impingesupon the cantilever, while at the same time minimizing the undesirablereflective interferences caused by extraneous waves. The length oflinear beam 60 is preferably a length equivalent to the width of thecantilever 62 plus a distance 64 extending from the edges of thecantilever 66 to account for mechanical misalignment tolerances.Specific values of the typical mechanical misalignment will vary inaccordance with a particular probe design, and may be obtained from amanufacturer. By sizing the linear beam in this way, a consistent forceis imparted to the cantilever, even when misaligned.

While some of wave energy from the linear beam towards the cylindricallens, providing a degree of undesirable buildup of standing waves due tomultiple reflections, most of incident energy falls outside of thecantilever, and is dispersed so as to minimize reflective buildup. Also,diffraction losses are greater in such a focused system, and round-tripacoustic reflections will result in smaller resonances than for anunfocused plane wave illumination.

It is appreciated that while the implementation of a cylindrical lensgreatly reduces undesirable reflection, some resonances due to reflectedwaves will still exist.

Particularly, it is understood that typically the roundtrip propagationtime for the return of the reflected radiation to cantilever is shorterthan the duration of the very narrow band (i.e., single frequency) RFtone bursts that are used for driving the actuator, so the reflected andthe non-reflected radiation incident on the cantilever coherentlyinterfere. This interference may be constructive or destructive, and theresonance conditions change as the spacing between the cantilever andacoustic source varies by as little as one quarter acoustic wavelength.These resonances will result in strongly varying acoustic pressure onthe cantilever for small changes in the spacing between the cantileverand acoustic source.

In accordance with the present embodiment, provision is made forsignificantly reducing the effect of resonances on the acoustic powerdensity of the acoustic beam or beams that are incident on thecantilever, which thereby reduce the accuracy of the detected readings.Approaches to accomplish this employ multi-frequency and/or employfrequency RF voltage pulses for driving the actuation so that theacoustic power perturbations caused by the resonances andanti-resonances of the different frequencies tend to neutralize eachother.

A first approach to diminish the effects of undesirable reflectance isto drive the actuator with multifrequency RF tone bursts, such that thepower perturbations caused by the resonances of one frequency componentsubstantially offset or neutralize the perturbations caused by theanti-resonances of another frequency component, and vice-versa. Moreparticularly, referring to the dual tone driver 70 illustrated in FIG.6, it will be understood that if the resonances of the lens substrate 30and of the transducer 16 are ignored, a cantilever location level atwhich one frequency, f₁, is resonant in the fluid arid another frequencyf₂, is anti-resonant can be determined as a function of thedisplacement, l_(i), of the cantilever from the central portion of thelens surface (i.e., the “acoustical center” of the lens 36).

If applied together to the acoustic source, the power perturbationscaused by the resonances and anti-resonances of f1 and f2 will tend toneutralize each other, thereby reducing the inaccuracies of the detecteddeflection values.

Alternatively, or in addition, the frequency content of the RF drivepulses may be increased. For example, as shown in FIG. 7, a mixer 80 maybe employed for mixing an RF carrier, such as a 150 MHz carrier, with acyclical pseudo-random bit sequence signal having a frequency of about20 MHz, such that the drive pulses that are applied to the transducer 16by a switch or modulator 82 are composed of a large number of RFfrequencies ranging from about 130 MHz to about 170 MHz. Suitably, thepseudo-random bit sequence signal is supplied by a pseudo-random bitgenerator 84 which cycles at the data rate of the activator (i.e., therate at which data bits are applied to the modulator 82), therebyensuring that the RF power of the drive pulses applied to the transducer16 is substantially uniform. It is understood the provided values aresimply used for illustration, and the specific values will be dependenton a particular implementation.

Alternatively, a linear chirp signal could be employed to modulate theRF carrier frequency, this embodiment is desirably implemented with thecarrier frequency modulated at a high rate. Still another alternative isto employ data modulated, essentially “white” RF noise, for driving thetransducer 16.

In the dual tone embodiment of FIG. 6, the amplitudes of the twofrequency components, f₁ and f₂, can be scaled as required to ensurethat their resonances and anti-resonances substantially equally andoppositely perturb the acoustic power at the cantilever. However, when abroad or spread spectrum RF source is employed, such as in FIG. 7, it issimpler to design the source so that it has a relatively flat amplitudeacross its entire frequency spectrum.

By implementation of these reduction techniques, undesirable reflectionsof FIGS. 5A, 5B are further reduced. Also, by implementing thesetechniques, a beam which encompasses, i.e., more fully covers andextends over the cantilever (or fully covers and extends over thecantilever, e.g., FIGS. 3A, 3B), may be used, as the reflectance signalswill be substantially neutralized.

As is apparent from the above discussion, the force imposed by thetransducer 16 is unidirectional. As a result, ultrasonic beam 34 cannotpull the cantilever 22 in the propagation direction. If it is desired toconfigure the AFM 10 to selectively pull the cantilever 22 toward thesubstrate 30 rather than push it away from the substrate, the cantilever22 could be manufactured with a bias that maintains it in contact withthe substrate 30 in the absence of a drive signal to the RF signalgenerator 32. The RF signal generator 32 could then be driven toovercome the bias and push the cantilever 22 from the substrate 30. Thedrive voltage could then be reduced to permit the cantilever 22 to movetowards the substrate 30, hence, in effect, pulling the cantilever 22towards the substrate 30. Alternatively, in some configurations, anultrasonic actuator could be placed both above and/or below thecantilever to push from one or both sides.

The previous discussion demonstrates a particular capability of thecurrent concepts. The ultrasonic cantilever actuator with a cylindricallens can independently control both the DC and AC forces applied to thecantilever over an extremely wide bandwidth. This opens up a range ofapplications for this actuator. One example is an imaging method calledForce Modulation. In this method, an AFM tip is brought into contactwith a sample surface and then an AC modulation force is applied to thecantilever. The detector then measures the amount of AC deflection ofthe cantilever. On hard samples, the cantilever cannot indent into thesurface, and no deflection is detected. On softer samples, theultrasonic modulation force causes the tip to indent into the sample,resulting in a measurable AC deflection of the cantilever. Separatecontrol over the DC force allows control over the tracking force thatthe AFM system uses to maintain contact between the tip and the surface.

The RF voltage can also be varied slowly to permit a quasistaticmeasurement to be performed. A quasistatic force imposition process isconsidered to be one in which, if the forces were to be removed at anystage during the process, the system would be in equilibrium from thattime on. Hence, the RF voltage can be changed slowly enough to maintainequilibrium while the voltage is being altered. This procedure is incontrast to a dynamic (AC) measurement in which the RF signal ismodulated at a high frequency and the system requires time to stabilizewhen force adjustment terminates. The cutoff between quasistatic anddynamic measurements is usually considered to be a frequency value belowthe cantilever's fundamental resonant frequency.

The relationship between cantilever deflection and drive voltagemagnitude is proportional and can be used to obtain useful informationconcerning a sample. For instance, the RF signal generator 32 of FIG. 1can be controlled to generate force curves. Force curves are often usedto provide an indication of the magnitude of force required to obtain aneffect such as indenting a sample surface, breaking the bindingmolecules between a sample and a probe in contact with the sample, etc.In order to generate a force curve with prior instruments, it wasnecessary to actuate a Z actuator in the scanhead to drive the probeagainst the sample (or move it away from the sample). The same effectcan be obtained using an ultrasonic actuator simply by modulating thedrive signal to an RF signal generator or other power source until thedesired effect is achieved. The known proportional relationship betweenthe signal drive voltage and cantilever deflection can then be used tocalculate the force. No actuation of a separate Z actuator is required.

AFM cantilevers can also respond dynamically to radiation pressure, andthose dynamic responses can be measured. Specifically, time harmonicforces can be generated by applying a sinusoidal amplitude modulation onthe RF input signal. By choosing the modulation factor to be less thanone, an appropriate biasing force can be applied to actuate thecantilever at the modulation frequency and its second harmonic. Thedeflection of the cantilever can then be recorded, e.g., by using alock-in amplifier, which uses the modulation signal as its referenceinput and locks to the modulation frequency.

It has been discovered that an ultrasonic-actuator based system has avery wide bandwidth for exciting the cantilever. In practice, it ispossible to modulate the RF signal up to about 1/10 of the RF signalfrequency or even higher. For a 300 MHz RF frequency, a cantileveractuation bandwidth of even 30 MHz is realizable. In fact, it isbelieved that the RF frequency from a low of 10 MHz or possibly evenlower for air applications to 1 GHz or even higher for waterapplications with surface micromachined cantilevers for whichattenuation may not be a problem. This cantilever actuation bandwidth ismuch greater than is provided by other AFM cantilever actuators,particularly acoustic and magnetic drives. An AFM having anultrasonically driven cantilever can therefore be used to scan at ratesthat would have theretofore been considered unobtainable.

An ultrasonic actuator of the type described above can be used to drivean AFM cantilever to oscillate at virtually any desired frequencysignificantly below the RF carrier frequency. An ultrasonic actuatortherefore can be directly used as a tapping mode actuator in an AFM. Atapping mode AFM 110 using an ultrasonic actuator is shown schematicallyin FIG. 8. As with the more theoretical embodiment of FIG. 1, theinstrument 110 includes a conventional probe 112, an ultrasonic actuator116, and a detector 120. The probe 112 is configured to operate in afluid cell 114 containing a sample 121. It includes a cantilever 122having a base affixed to a support 126 and a free end bearing a tip 124.Also as in the embodiment of FIG. 1, an ultrasonically transmissivesubstrate 130 is placed below the cantilever 122. A cylindrical lens 129such as a linear Fresnel lens is placed on or in the substrate 130. Useof cylindrical lens 129 will cause a beam 131 having control in a singledimension. The ultrasonic actuator 116 is mounted on the bottom of thesubstrate 130 and powered by an RF signal generator 132 under thecontrol of an AFM controller 138. The RF signal generator 132 producesan RF oscillation in the ultrasonic actuator 116 and then modulates theamplitude of that signal in response to a tapping mode drive signal assupplied by the controller 138. The controller 138 may turn the drivesignal on and off with a square wave, or it could modulate the amplitudeof the drive signal in proportion to that sine wave. The controller 138also drives an XYZ actuator in the scanhead 128 in the conventionalmanner.

The traditional tapping mode piezoelectric drive may be taken out of theloop and replaced by the ultrasonic actuator 116. In this case, the RFdrive signal described in the preceding paragraph would always be usedto drive the ultrasonic actuator 116. In the preferred embodimenthowever, a piezoelectric drive 140 can be retained, and a suitableswitch 142 can be provided to permit a drive signal to be selectivelytransmitted to either modulate the output of RF signal generator 132 andhence activate the ultrasonic actuator, or the drive signal can be sentto the piezoelectric drive 140 directly from the AFM controller 138. Forexample, the resulting instrument could be operated in either air orliquid, with the piezoelectric drive 140 being used to effect operationin air and the ultrasonic actuator 116 being used to effect operation inliquid.

An issue of the instruments previously illustrated is that the sample isultrasonically transmissive to permit unfettered transmission of theultrasonic beam from the ultrasonic actuator, through the sample, and tothe cantilever. A more versatile ultrasonic actuator assembly isschematically illustrated in FIG. 9. In this embodiment an ultrasonicactuator is mounted on a holder 160 positioned above the cantilever 162.The holder 160 is also positioned between the cantilever 162, on onehand, and a laser 164 and photodetector (not shown), on the other hand.The holder 160 may be constructed of glass or any other material that istransparent to light and transmissive to ultrasonic energy. In theillustrated embodiment, a ZnO transducer 166 is mounted on the holder160. A cylindrical lens 168, such as a linear Fresnel lens, is formed inor mounted on the holder 160. The relevant portions of the holder areinclined to direct the corresponding ultrasonic beam 170 as a singledimension in the lateral direction of the longitudinal of the cantilever160.

An instrument 210 configured for elasticity characterization isillustrated in FIG. 10. It includes all of the components of the AFM ofFIG. 8, including probe 212, a fluid cell 214, an ultrasonic transducer216, a scanhead 218, a substrate 220 (with a cylindrical lens 22, builtin or located on top thereof), and a piezoelectric drive 222. Use ofcylindrical lens 221 causes wave 223 to be generated. The probe 212includes a cantilever 224 that has a base mounted on a support 226 andthat has a free end bearing a tip 228. Electronic components of theinstrument 210 include an RF signal generator 230, an AFM controller232, and a switch 234 selectively coupling the controller 232 to the RFsignal generator and the piezoelectric drive 222. They additionallyinclude a lock-in amplifier 236 and a low frequency signal generator238. The lock-in amplifier 236 receives a feedback signal from the lowfrequency signal generator 238 and transmits an elasticity image signalto the AFM controller 232. Using conventional feedback to keep the forceapplied to the sample 240 constant, an image can be formed by monitoringcantilever deflection at the modulation frequency. The image, taking theform of an AC signal, has an amplitude and phase that both vary as afunction of the sample stiffness. The procedure therefore can yield twosimultaneous images: one for topography and one for surface elasticity.Elasticity characterizations using this technique can be performed muchmore rapidly than with prior known techniques due to the fact that theultrasonic actuator has a dramatically higher bandwidth than priorsystems that relied on a piezoelectric actuator to drive the entireprobe up and down to obtain the required measurements. The prior systemstypically had a maximum modulation frequency of only a few tens of kHz,whereas (as discussed above) an ultrasonic actuator based system has acantilever actuation bandwidth of several MHz.

While the discussion has described the invention primarily incorporatedinto AFMs, it is to be understood the invention is not limited to thedescribed embodiments or even to AFMs in general. Rather, it isapplicable to virtually any probe-based instrument in which a cantileveris deflected by directing a focused beam of acoustic energy at thecantilever to apply ultrasonically generated acoustic radiation pressureto the cantilever. A variety of different ultrasonic acoustic actuatorsand associated drives may be applied to achieve these affects, andcantilever deflection may be measured using a variety of techniques.Also, while there is particular usefulness for use of cantilevers influid, such as water embodiments, other fluids may also take advantageof the present applications, including those in gases, the air or othernon-water liquids as well as use in vacuums.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. A probe-based instrument comprising: a cantilever; an acoustic actuator configured to direct a beam of acoustic energy at the cantilever so as to drive the cantilever; and an RF excitation signal passed to the acoustic actuator to generate the beam of acoustic energy, the RF excitation signal being spread over a frequency spectrum to reduce resonance effects of the signal.
 2. The probe based instrument of claim 1, wherein the RF excitation signal is generated by a signal generator.
 3. The probe-based instrument according to claim 1 wherein the beam of acoustic energy is substantially a linearly focused beam.
 4. The probe-based instrument according to claim 1, wherein the actuator includes a cylindrical lens, to generate the substantially linearly focused beam.
 5. The probe-based instrument according to claim 1, wherein the beam is sized to extend over a width of the cantilever, and a distance from each cantilever edge sufficient to encompass the cantilever when the cantilever is misaligned.
 6. The probe-based instrument according to claim 1, wherein the cantilever is a scanning probe microscope cantilever.
 7. The probe-based instrument according to claim 1, wherein the cantilever is an atomic force microscope cantilever.
 8. The probe-based instrument according to claim 1, wherein the cantilever is driven to oscillate at the resonant frequency of the cantilever or a harmonic thereof.
 9. The probe-based instrument of claim 1, wherein the RF excitation signal spread over the frequency spectrum is a spread frequency oscillation signal and is comprised of RF voltage pulses, configured so that acoustic power perturbations generated by the RF pulses have resonances and anti-resonances which act to neutralize each other, to provide the diminishing of the undesirable resonance effects.
 10. The probe-based instrument of claim 1, wherein the RF excitation signal spread over the frequency spectrum is a spread frequency oscillation signal generated by a signal generator and is formed by at least one of multifrequency RF tone bursts, an RF drive signal with increased frequency content generated by a mixer for mixing an RF carrier signal with a cyclical pseudo-random bit sequence signal, a linear chirp signal or modulated white noise.
 11. The probe-based instrument according to claim 1, further configured as an Atomic Force Microscope (AFM), including an AFM tip.
 12. The probe-based instrument according to claim 11, wherein the AFM is operated in a Force Modulation mode.
 13. The probe-based instrument according to claim 11, wherein the AFM is operated in a tapping mode.
 14. The probe-based instrument according to claim 1, wherein the frequency spectrum is from 10 MHz to 1 GHz.
 15. The probe-based instrument according to claim 1, wherein the RF signal is modulated up to 1/10 of the RF signal frequency.
 16. The probe-based instrument according to claim 1, further configured to determine elasticity characterization.
 17. The probe-based instrument according to claim 1, further configured at least partially within a liquid.
 18. A method of moving a cantilever of a probe-based instrument comprising: generating an RF excitation signal; passing the RF excitation signal to an acoustic actuator; and generating a beam of acoustic energy, from the RF signal, at the cantilever so as to drive the cantilever, wherein the RF excitation signal is spread over a frequency spectrum to reduce resonance effects of the signal.
 19. A probe-based instrument comprising: a cantilever; an acoustic actuator configured to direct a beam of acoustic energy at the cantilever so as to drive the cantilever; and an RF excitation signal passed to the acoustic actuator to generate the beam of acoustic energy, the RF excitation signal being spread over a frequency spectrum to drive the actuation so that acoustic power perturbations caused by resonances and anti-resonances of different frequencies from the acoustic actuator act to neutralize each other.
 20. The probe-based instrument of claim 19, wherein the RF excitation signal is generated by a signal generator. 